add death-test for “create before registrationComplete()” guard (#2429)

Summary:
Pull Request resolved: #2429

# Motivation
------------------
* **Real-world failure case** – An HHVM extension recently crashed because its
  global `folly::Singleton` objects were first instantiated while the vault was
  still in the *registration* phase.  That path calls
  `singletonWarnCreateBeforeRegistrationCompleteAndAbort()` and raises
  SIGABRT.  The new test is a minimal repro so anyone investigating similar
  traces can run `singleton_test` and see the exact behavior.

* **Document the API contract** – Folly’s design intentionally treats
  “create-before-registrationComplete” as a hard logic error. A unit-test makes
  this behavior explicit and discoverable.  If maintainers later decide to soften or
 eliminate the abort, the failing test will prompt a conscious, reviewed decision
 instead of a silent behavior change.

# What the test does
------------------
1. Creates a *fresh* vault identified by a private `VaultTag`; a new vault starts
   in the **REGISTRATION** state.
2. Registers a dummy singleton `<Dummy, Tag, VaultTag>` against that vault.
3. Calls `try_get()` **before** `registrationComplete()` is invoked.
4. Uses `EXPECT_DEATH` to assert that the process aborts with
   `singletonWarnCreateBeforeRegistrationCompleteAndAbort`.

Reviewed By: yfeldblum

Differential Revision: D73976074

fbshipit-source-id: eaaff85669c39d4cebb9923f0de3431ea6c19f7f

Author: Liu Cao

folly/Singleton.h

@@ -129,6 +129,11 @@
 /// Singletons won't be recreated after destroyInstances call. If you
 /// want to re-enable singleton creation (say after fork was called) you
 /// should call reenableInstances.
+///
+/// NOTE: Calling `try_get()` **before**
+/// `SingletonVault::registrationComplete()` has completed (i.e. before
+/// `folly::init()` completed) is a logic error. In this case, `try_get()`
+///  will abort via `singletonWarnCreateBeforeRegistrationCompleteAndAbort()`.
 /// @class folly::Singleton
 
 #pragma once

folly/test/SingletonTest.cpp

@@ -1232,3 +1232,18 @@ auto cancelOnDestructionSingleton =
 TEST(Singleton, CancelOnDestruction) {
   cancelOnDestructionSingleton.try_get();
 }
+
+TEST(Singleton, CreationBeforeRegistrationCompleteAborts) {
+  struct Dummy {};
+  struct VaultTag {};
+  struct Tag {};
+
+  folly::SingletonVault* vault = folly::SingletonVault::singleton<VaultTag>();
+  std::ignore = vault;
+
+  static folly::Singleton<Dummy, Tag, VaultTag> lateSingleton;
+
+  EXPECT_DEATH(
+      { std::ignore = lateSingleton.try_get(); },
+      "singletonWarnCreateBeforeRegistrationCompleteAndAbort");
+}